This invention relates generally to turbofan engines, and more particularly to a method and apparatus for reducing the operational noise level of the turbofan engine.
Turbofan gas turbine engines typically include a fan assembly that channels air to a core gas turbine engine and to a bypass duct. The core gas turbine engine includes a high-pressure compressor, a combustor, and at least one turbine. The compressor compresses airflow channeled from the fan assembly and delivers the compressed air to the combustor where it is mixed with fuel. The mixture is then ignited for generating hot combustion gases. The combustion gases are channeled to the turbine(s) which extracts energy from the combustion gases for powering the compressor(s), as well as producing useful work to propel an aircraft in flight.
During operation, the fan operates similarly to a propeller by supplying air downstream to the gas turbine engine and also pushing air through the exhaust nozzle at an increased velocity to provide thrust for the aircraft. As such, airflow exiting the fan is split into two different separate paths generally referred to as the fan duct stream and the core duct stream. More specifically, the fan imparts a swirling motion to the air prior to the air being channeled through the fan duct. This swirling motion may cause a loss of momentum before the air exits the exhaust nozzle. Accordingly, at least some known turbofan engines include a set of stator vanes to facilitate reducing the air swirling motion before the bypass duct. However, the airflow impacting the stator vanes may cause an increase in noise emissions. For example, fan air impacts the stator vanes at the rate of blades passing by and generates a tone often referred to as the blade passage frequency (BPF). Moreover, the unsteadiness in the fan flow may interact with the stators to create broadband noise. As larger turbofan engines with higher bypass ratios are produced to address the need for increased thrust requirements at low levels of specific fuel consumption, the fan becomes the predominant source of noise emanating from the engine. Several attempts have been made to reduce fan noise in turbofan engines as the international recognition of the need to limit or reduce engine noise level increases, particularly for communities living near airports. However, known attempts to address this important problem have involved indirect remedies that do not directly affect the source of the problem with clear undesirable side effects, such as added engine weight. Therefore, a need exists for turbofan engines producing a substantially reduced level of noise while maintaining desired performance levels at potentially reduced overall engine weight.